It’s a warm summer evening and all the car guys are out on the town. Through the night air carries the sound of a sweet-running small-block crisply hitting that wide-open-throttle 1-2 shift. You anticipate that unseen gear-jammer hitting that critical 2-3 shift when the engine suddenly zings through its missed shift. There is a collective groan and the wiser ones just shake their head. And it’s almost guaranteed the perpetrator will remain anonymous—unless his friends rat him out!
Oftentimes, the guy pulling the handle is not always to blame. Sometimes it’s the equipment—or the way it was installed—that’s the real culprit. The TREMEC TKO-500 and -600 row-your-own gearboxes have been a big favorite in the retrofit scene for many years. These five-speed manuals have a lot going for them. Besides being affordable, they’re also relatively lightweight with a compact size that makes it easy to squeeze them under just about any muscle car floorpan.
With all this going for them, the TREMEC is not without a few minor issues. Among the most prevalent is a subjective notchy shifter feel, even with low-rpm gear changes. Some claim it is near unwilling to complete upshifts at speeds beyond 6,500 rpm. This is often aggravated by retrofit shifter relocation techniques. The tolerance stack-up of these idiosyncrasies has all the makings of generating a bad rap for a great transmission. Rather than condemn the transmission, a better approach is to use a few blueprinting techniques to make the transmission and shifter work for you instead of against you.
There are several conditions that can create poor shifting performance that have nothing to do with the transmission or the shifter. Bellhousing misalignment, poor driveshaft angles that can cause vibrations, and poor clutch linkage setup will all affect shift performance so we have to be fair to the trans and say that not all problems are related just to the box.
Luckily there are a few hardy manual transmission specialists who have persevered to discover the keys to delivering a smoother shift even at very high engine speeds. This story will focus on one such balky TKO-600 that has taken up residence in a ’66 Chevelle. The original conversion was performed several years ago with limited success. The issue was that the shifter was just plain balky and rapid shifts at engine speeds above 5,500 rpm were suspect. The whole system needed attention. That’s when we contacted Don Walsh, Sr. at D&D Performance.
Walsh and his crew at D&D has been improving the world of manual transmissions for several decades so they have a pretty good handle on how to make gears mesh properly. Walsh says the TREMEC is a great transmission but he thinks it suffers from a synchronizer originally designed for the Ford Top Loader four-speed designed back when JFK was president over a half-century ago. D&D has studied this transmission and come up with some simple modifications that can smooth out its balkiness and make shifting a much more pleasurable experience—especially when the revs approach 7,000!
Bill Irwin’s ’66 small-block Chevelle isn’t aimed at quicker dragstrip times, so D&D didn’t perform its custom drag race synchro tooth modification. D&D did, however, add its upgraded synchro rings and also modified the shift lugs to make the 2-3 shift less clunky. The remainder of the rebuild replaced the bearings and ensured the box was in optimal shape.
With the trans was out, Irwin decided a new flywheel, clutch, and pressure plate was also in order. D&D prefers to work with systems from Ram Clutches and since this Chevelle is spinning a relatively mild 383, Walsh recommended the DXH series single-disc clutch assembly along with new fasteners.
While the trans was luxuriating at D&D rehab, Irwin spent the time installing the new clutch assembly, which also meant checking the bellhousing alignment. Because the TREMEC employs a tapered roller bearing on the input shaft, it is less tolerant of bellhousing misalignment compared to ball bearing transmissions like the Muncie. Irwin discovered his original scattershield was 0.018-inch out (the maximum misalignment spec is 0.006-inch), so he used a set of McRobb 0.007-inch offset dowel pins to bring the bellhousing within 0.003-inch of perfection. The point here is that even the best trans will not shift properly if the crankshaft and transmission centerlines are not aligned.
TREMEC and D&D also recommend using either GM’s Synchromesh or Pennzoil’s equivalent fluid. Walsh says they have had great luck with both fluids, but warns not to use ATF as this can cause damage to the bearings and gears. Synchromesh fluid is readily available in many retail outlets, including Summit Racing.
We’ll cover the D&D resuscitation of this TKO-600 and reveal how a few simple modifications can convert this sluggish gearbox into a gearbox that almost anticipates your next shift. The modifications aren’t difficult but do require shipping the transmission to D&D in Wixom, Michigan. There will be some shipping costs involved but the result will be well worth the effort. And then you can be that gearbox hero who completes all three of those high-rpm gear changes on that next warm, Saturday night.
1. Your tactile connection to the transmission is all through the shift handle. If the trans won’t shift, rowing through the gears is just no fun. D&D’s TKO mods put the fun back into your TREMEC TKO trans.
2. This is our TKO-600 just as we pulled it out of the Chevelle. It shifted acceptably at low speeds but balked, especially with the 2-3, at anything over 5,500 rpm. We later learned that this was partially due to misguided shifter relocation and to bad bellhousing alignment.
3. After disassembling the trans, D&D found it in decent condition with no broken parts. The original TKO brass synchros (left) are partially the cause of this gearbox’s shifting woes. D&D replaces them with carbon-fiber weave internal sleeve synchros (right). The carbon fiber is more aggressive at slowing the gear, completing the shift in less time.
4. One minor change that improves shift quality is replacing the stock plastic shift fork pads with bronze versions. The TKO-500/600 uses a total of nine pads for a complete transmission. This is the 1-2 fork with three pads while the Fifth gear fork uses only two. Walsh says some early TKOs used two pads on the 1-2 and 3-4 forks.
5. The most common issue with the TKOs is hard shifting above 6,000 rpm. To address this, D&D removes every other tooth on both the 1-2 and 2-3 synchros to create an easier path to complete the gear change. This modification can reduce synchro life, so this modification is generally reserved for fast street cars or pure race boxes.
6. D&D also modified the 3-4 shift lug to make the 3-4 shift much cleaner. The stock shift arm is on the right. D&D modifies the arm (left) by welding on more material and then adding a steep bevel.
7. Before reassembly, D&D tumbled the case and extension housing and then carefully reassembled the trans. The case now appears almost new—but we’ll hold off on putting mirrors under the car at shows.
8. To ensure proper clutch release and easier shifting, Irwin checked the concentricity of the bellhousing and found it was out of alignment by 0.018-inch. This required a set of 0.007-inch offset dowel pins (that we got from McRobb Performance) to get the bellhousing within the 0.006-inch spec. Parallelism of the mounting face was only 0.006-inch so it did not require machining.
9. These are the McRobb offset dowels and as far as we are concerned the only way to dial in your bellhousing. The split allows you to slip the dowels in place and easily rotate them into position. Once the dowels are properly located and the bellhousing is aligned, tightening the small Allen bolt expands the split and locks the dowel in place; a very slick idea.
10. As part of the upgrade, Irwin also replaced the old clutch, pressure plate, and flywheel with a complete Ram HDX street clutch system. Irwin’s 383 makes around 450 hp so this clutch package offers both good holding power while keeping the pedal effort low.
11. First design pressure plates use shouldered bolts to accurately center the pressure plate to the flywheel. Shouldered bolts are shown here on either side of a standard bolt. Never use standard bolts for a shouldered bolt application as this could cause a vibration from an off-center pressure plate. We used Ram shouldered pressure plate bolts.
12. Irwin used a Ram steel dummy shaft to align the clutch so the trans will slide easily into place. If the trans won’t bottom out against the bellhousing do not try to pull the trans case tight with the bolts. That will crack or break the mounting ears. Instead, install the clutch linkage and have a friend depress the clutch pedal while you push on the trans. When the pressure plate releases load on the clutch, the trans will slide right in place.
13. Bill prefers to use homemade guide pins in the top two trans bolt holes to guide the trans into place. The guide pins prevent hanging the input shaft on the clutch and perhaps misaligning it or bending the clutch hub—which can happen very easily. These guide pins make installing a trans much easier.
14. With its upgraded TKO, Bill’s Chevelle is now back on the road with its much smoother shifting overdrive trans making it well worth the effort!
|TREMEC TKO-600||TA600C||D&D Perf.|
|D&D blocker ring, carbon||TS1L||D&D Perf.|
|D&D TKO shift fork pad||TS42B||D&D Perf.|
|Modified 2-3 shift finger||TS28M||D&D Perf.|
|Labor to Upgrade TKO-500/600||Call||D&D Perf.|
|Ram steel 11-inch, 168-tooth flywheel||1501||D&D Perf.|
|Ram HDX series clutch, pressure plate||88764HDX||D&D Perf.|
|Ram pilot bushing||C2229||D&D Perf.|
|D&D flywheel bolts, need 6||C88||D&D Perf.|
|D&D pressure plate bolts, set of six||C490||D&D Perf.|
|Slip yoke for TKO-600||D70||D&D Perf.|
|Ram 26-spline clutch alignment tool||03-013||Summit Racing|
|McRobb 0.007 offset dowel set||1011||McRobb|
|McRobb 0.014 offset dowel set||1012||McRobb|
|McRobb 0.021 offset dowel set||1013||McRobb|
|Pennzoil Synchromesh fluid, 1 Qt.||3501||Summit Racing|
|ACDelco Synchromesh trans fluid, 32oz||88900399||Summit Racing|
Photography by Jeff Smith